From what I can tell so far, Gaal has been modified to basically be a combination of the original Gaal character, Manella Dubanqua, and potentially Wanda Seldon. They've given this Gaal a backstory of her own. We're in a period of time after the second episode that isn't covered by the books pretty much at all. The events of the prequel books end two years before the trial we see in the first episode. We get the trial. Aside from an epilogue short on detail, the next writing takes place 50 years later. If they weren't going to explore the prequel books, then I think merging those 2-3 characters makes sense from a narrative point-of-view. You can't have the story without including Wanda's part and you can't have Wanda without Manella.
To this point, there's very little that can't be explained by the original stories being told as legend and so are these, thus some timing and minutae differences. The only thing I'm nervous about is The Vault, which is clearly far different.
